docs(docs): add frontmatter to all design docs

Standardized YAML frontmatter on all 40 docs/design/ files with title,
description, type, status, ticket, decision_refs, and author fields.
Covers specs, wireframes, voice cards, spatial layouts, and system
designs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/design/content-directory-structure.md

@@ -1,3 +1,15 @@
+---
+title: "Content Directory Structure"
+description: "Source of truth for the content/ directory layout, schemas, validation pipeline, and mod-compatible conventions"
+type: design
+status: active
+ticket: "#384"
+decision_refs: [D-024, D-027, D-028, D-032, D-034, D-035, D-036, D-049, D-057]
+author: ""
+created: 2026-02-19
+updated: 2026-02-19
+---
+
 # Content Directory Structure — The Settled Reach v0.1
 
 **Ticket:** #384 (blocks #385: directory skeleton, #386: schema definitions)